The subject matter of the invention is a process for the recovery of S-(carboxymethyl)-(R)-cysteine and S-(carboxymethyl-(S)-cysteine from a mixture of the two enantiomers.
S-(carboxymethyl)-cysteine, especially the (R)-enantiomer, has significance as a pharmaceutically active material and is also employed in the cosmetic industry, for example, in the production of hair setting lotions.
For a long time S-(carboxymethyl)-(R)-cysteine was obtained from natural keratin containing raw materials. These raw materials were hydrolyzed and worked up to (R)-cysteine which then subsequently according to the process known from Armstrong, J. Org. Chem. Vol. 16 (1951), pages 749 to 753 could be reacted in alkaline medium with chloroacetic acid to form S-(carboxymethyl)-(R)-cysteine. However, suitable natural raw materials are only available to a limited extent.
In the meantime, however synthetic (RS)-cysteine has become readily accessible, for example, according to the known process from Angew. Chem. Vol. 93 (1981), pages 680 and 683 by way of 2,2-dimethyl-3-thiazoline and 2,2-dimethylthiazolidine-4-carbonitrile. Its reaction with chloroacetic acid then furnishes S-(carboxymethyl)-(R,S)-cysteine. There exists a need to separate this racemate into the two optically active enantiomers, by an efficient resolution process.